Sixth normal form
Encyclopedia
Sixth normal form is a term in relational database
theory, used in two different ways.
and others on temporal database
s, defined sixth normal form as a normal form for databases based on an extension of the relational algebra.
In this work, the relational operators, such as join, are generalized to support a natural treatment of interval data, such as sequences of dates or moments in time. Sixth normal form is then based on this generalized join, as follows:
Any relation in 6NF is also in 5NF
.
Sixth normal form is intended to decompose relation variables to irreducible components. Though this may be relatively unimportant for non-temporal relation variables, it can be important when dealing with temporal variables or other interval data. For instance, if a relation comprises a supplier's name, status, and city, we may also want to add temporal data, such as the time during which these values are, or were, valid (e.g., for historical data) but the three values may vary independently of each other and at different rates. We may, for instance, wish to trace the history of changes to Status.
For further discussion on Temporal Aggregation in SQL, see also Zimanyi. For a non-relational approach, see TSQL2.
(DKNF). This usage predates Date et al.'s work.
s where the benefits outweigh the drawbacks, for example using Anchor Modeling
. Although using 6NF leads to an explosion of tables, modern databases can prune the tables from select queries (using a process called 'table elimination') where they are not required. Queries that only access several attributes will then be faster than similar queries in databases modelled in the Third normal form
.
Relational database
A relational database is a database that conforms to relational model theory. The software used in a relational database is called a relational database management system . Colloquial use of the term "relational database" may refer to the RDBMS software, or the relational database itself...
theory, used in two different ways.
6NF (C. Date's definition)
A book by Christopher J. DateChristopher J. Date
Chris Date is an independent author, lecturer, researcher, and consultant, specializing in relational database theory.-Biography:Chris Date attended High Wycombe Royal Grammar School from 1951 to 1958 and received his BA in Mathematics from Cambridge University in 1962. He entered the computer...
and others on temporal database
Temporal database
A temporal database is a database with built-in time aspects, for example a temporal data model and a temporal version of Structured Query Language.More specifically the temporal aspects usually include valid-time and transaction-time...
s, defined sixth normal form as a normal form for databases based on an extension of the relational algebra.
In this work, the relational operators, such as join, are generalized to support a natural treatment of interval data, such as sequences of dates or moments in time. Sixth normal form is then based on this generalized join, as follows:
A relvar R [table] is in sixth normal form (abbreviated 6NF) if and only if it satisfies no nontrivial join dependencies at all — where, as before, a join dependencyJoin dependencyA join dependency is a constraint on the set of legal relations over a database scheme. A table T is subject to a join dependency if T can always be recreated by joining multiple tables each having a subset of the attributes of T...
is trivial if and only if at least one of the projections (possibly U_projections) involved is taken over the set of all attributes of the relvar [table] concerned.[Date et al.]
Any relation in 6NF is also in 5NF
Fifth normal form
Fifth normal form , also known as Project-join normal form is a level of database normalization designed to reduce redundancy in relational databases recording multi-valued facts by isolating semantically related multiple relationships...
.
Sixth normal form is intended to decompose relation variables to irreducible components. Though this may be relatively unimportant for non-temporal relation variables, it can be important when dealing with temporal variables or other interval data. For instance, if a relation comprises a supplier's name, status, and city, we may also want to add temporal data, such as the time during which these values are, or were, valid (e.g., for historical data) but the three values may vary independently of each other and at different rates. We may, for instance, wish to trace the history of changes to Status.
For further discussion on Temporal Aggregation in SQL, see also Zimanyi. For a non-relational approach, see TSQL2.
DKNF
Some authors use the term sixth normal form differently, namely, as a synonym for Domain/key normal formDomain/key normal form
Domain/key normal form is a normal form used in database normalization which requires that the database contains no constraints other than domain constraints and key constraints....
(DKNF). This usage predates Date et al.'s work.
Usage
The sixth normal form is currently being used in some data warehouseData warehouse
In computing, a data warehouse is a database used for reporting and analysis. The data stored in the warehouse is uploaded from the operational systems. The data may pass through an operational data store for additional operations before it is used in the DW for reporting.A data warehouse...
s where the benefits outweigh the drawbacks, for example using Anchor Modeling
Anchor Modeling
Anchor Modeling is an agile database modeling technique suited for information that change over time both in structure and content. It provides a graphical notation used for conceptual modeling similar to that of Entity-Relationship modeling, with extensions for working with temporal data...
. Although using 6NF leads to an explosion of tables, modern databases can prune the tables from select queries (using a process called 'table elimination') where they are not required. Queries that only access several attributes will then be faster than similar queries in databases modelled in the Third normal form
Third normal form
In computer science, the third normal form is a normal form used in database normalization. 3NF was originally defined by E.F. Codd in 1971. Codd's definition states that a table is in 3NF if and only if both of the following conditions hold:...
.